The invention relates to a disc reading system including a disc cassette and an apparatus into which the cassette is introduced. The disc cassette includes a rotatable disc, an external sleeve with a front opening which communicates with a disc-receiving space, an internal disc holder comprising a shutter for closing the front opening of the sleeve, and a pair of retaining means for retaining the disc and the disc holder as a unit inside the sleeve. The retaining means are movable relative to each other between a retaining position in which the disc is retained and a free position in which the disc is free to move relative to the disc holder. Latching means are movable with the aid of external means from a latching position in which the disc holder is latched in the sleeve and a free position in which the disc holder can be slid out of the sleeve. The apparatus includes disc-cassette release means for releasing the disc holder of an introduced disc cassette, and an extraction mechanism for extracting the disc-holder/disc unit from the sleeve and retaining the unit inside the apparatus during withdrawal of the sleeve.
Systems of this type are in use in which the disc is provided with video information which can be read by capacitive means and can be reproduced by means of a television set (see for example U.S. Pat. No. 4,361,884, hereby incorporated by reference). In these known video-disc systems it is desirable that the video disc is always, i.e. both when situated inside and outside the apparatus, protected against dust and touching. Nevertheless it should be easy to play the disc inside the apparatus. By accommodating the video disc in a disc cassette and removing the sleeve after insertion of the disc cassette into the apparatus the video disc can be played in the protective environment inside the apparatus. After playing, the sleeve is reinserted into the apparatus and the video disc is thus slid into the sleeve, so that after latching of the disc holder in the enclosure an easy-to-handle disc cassette is obtained, which adequately protects the video disc outside the apparatus.
U.S. Pat. No. 4,463,850 (hereby incorporated by reference) describes a disc-cassette intended for a system of the type defined in the opening paragraph. In this known disc cassette the retaining means of the internal disc holder comprise two jaws, at radially opposite portions of the periphery of the disc, which are free at one end and which are integral with the shutter at the other end. This unit is made of plastic and comprises two latching members which are integral with the jaws for latching the shutter in the side walls of the sleeve. The plastics jaws can deflect elastically between a position in which the disc is clear of the shutter and can thus be rotated and a position in which the jaws are deflected towards one another against the elasticity of the plastic to retain the disc in this position. This movement of the jaws towards one another is obtained by cooperation with the innerside of the two side walls of the sleeve at the instant at which the sleeve takes up the assembly comprising the shutter and the disc. If this assembly is again removed from the sleeve, the two jaws move outwards under the influence of the elasticity of the plastics, thereby releasing the disc.
A system comprising this known disc cassette has certain disadvantages. Plastic is known to exhibit some degree of relaxation under load, i.e., the elastic reactive force exerted by the plastic part decreases in the course of time. Thus, under the influence of a load the plastics may become permanently deformed in the long run. Some plastics have the property that after removal of the external load the deformation produced during relaxation is gradually eliminated partly or wholly, so that eventually the relevant part at least partly resumes its original position and its original resilience is restored.
As a disc cassette intended for a system as defined in the opening paragraph must be suitable for prolonged storage, it is not unlikely that as a result of relaxation the jaws are deformed to such an extent that after the disc holder/disc unit has been extracted from the sleeve and is retained in the apparatus the disc is no longer completely released from the disc holder. This may seriously disturb the operation of the apparatus, to such an extent that it is no longer possible to record or read information on/from the disc. Depending on the construction of the apparatus this may even lead to a permanent damage to the disc. In some known systems of the type intended here the apparatus employs mechanisms for lowering or lifting the disc out of the plane of the disc holder. If the disc holder obstructs this movement of the disc, the disc may be damaged or deformed. In particular if the disc contains computer information permanent damaging of the disc may have very undesirable consequences.
Another disadvantage of the known disc cassette is that if the disc holder together with the disc is removed from the sleeve outside the apparatus, on purpose or inadvertently, the disc may fall and be damaged. A further disadvantage is that when the disc cassette is closed the jaws press resiliently against the side walls of the sleeve which is generally also made of plastic. Thus, in the long run it is possible that the sleeve is deformed, which may give rise to problems when loading the disc cassette into the apparatus. A rattle free enclosure of the disc in the disc cassette is not guaranteed and is in fact impossible. The dimensions of the various parts which cooperate with each other can never be adapted to each other in such an accurate manner that the disc is situated in the disc holder and the disc holder in the sleeve exactly without any clearance between the disc holder and the disc and without large stresses being exerted on the disc periphery. Large stresses are very undesirable because they may lead to such a deformation of the disc that it can no longer be played. Moreover, the likelihood of deformation of the side walls of the sleeve increases substantially and is, in principle, even inevitable.